Switching circuit for an electrically heated blanket



Jam 1963 A. E. F. FICKWEILER 3,361,

SWITCHING CIRCUIT FOR AN ELBCTRICALLY HEATED BLANKET Filed April 20, 1965 IN VEN TOR.

AUGUST E. F. FICKWEILER BY 02% ATTORNEYS United States Patent 3,361,941 SWITCHING CIRCUIT FOR AN ELECTRICALLY HEATED BLANKET August E. F. Fickweiler, Waddinxveen, Netherlands, assignor to Indoheem, N.V., a corporation of Netherlands Filed Apr. 20, 1965, Ser. No. 449,462 Claims priority, applicationslzesglerlands, May 6, 1964, 2 Claims. (Cl. 317-132) The invention relates to a switching circuit for an electrically heated blanket, which is controlled by one or more negative temperature coeflicient resistors which are arranged in the blanket.

In such a switching circuit, the heating circuit is switched on through a relay as soon as the resistance of the negative temperature coefiicient resistors, which serve as temperature sensors, rises above a predetermined value, whereas the heating circuit is disconnected as soon as this resistance falls below a predetermined value, whereby the blanket temperature is automatically stabilized between two adjustable values in close proximity to each other.

Since the switching circuit is connected to the blanket through a flexible cord, it is not an imaginary hazard that a short-circuit or an interruption occurs in that cord or in the blanket itself. On the occurrence of a short-circult of the negative temperature coefiicient resistors, the resistance thereof falls below the predetermined value, in which case the heating circuit is disconnected, so that this cannot give rise to hazards. On the occurrence of an interruption of the negative temperature coeflicient resistors, however, the resistance thereof rises above the predetermined value, in which case the heating circuit is switched on, regardless of the blanket temperature, which can lead to scorching of the blanket or even to injury of the user.

It is an object of the invention to provide a control circuit of the type set forth above, in which the heating circuit is disconnected as soon as an interruption of the negative temperature coefficient resistors occurs.

For that purpose, it is a feature of a device according to the invention, that the heating circuit is connected to the supply voltage through a normally open contact of a relay, the coil of which, which is bridged by a capacitor, is connected in the main discharge circuit of a gas discharge valve with two control electrodes, which is supplied with an AC. voltage, the first control electrode being connected through the negative temperature coefficient resistor(s) to the positive terminal of a DC. voltage source and being connected through a conventional resistor to the negative terminal thereof, whilst the second control electrode is through conventional resistors connected to the positive and negative terminals respectively of that DC. voltage source, Whilst the first control electrode is also connected to the cathode of the gas discharge valve through a conventional grid leak with a resistance which is large as compared with the remaining resistors.

It the blanket temperature falls below the desired value, the resistance of the negative temperature coeificient resistors rises to such extent that a positive voltage with respect to the first control electrode and the cathode, which is connected to the first control electrode through a resistor, is applied to the second control electrode of the gas discharge valve, so that the valve ignited, the relay is energized and the heating circuit is closed.

The capacitor across the relay coil serves for smoothing the energizing current of the relay notwithstanding the AC. voltage supply.

If the blanket temperature rises above the desired value, the resistance of the negative temperature coeflicient resistors rises to such an extent, that the voltage at the second control electrode of the gas discharge valve is insuflicient for igniting the valve anew after the next passage through zero of the supply voltage, so that the relay is de-energized and the heating circuit is disconnected.

If an interruption of the negative temperature coeificient resistors occurs, the first control electrode is rendered negative to such an extent by the current flowing through the grid leak, that notwithstanding the positive voltage which is applied to the second control electrode, the gas discharge valve cannot ignite anew after the next passage through zero of the supply voltage, so that the relay is de-energized and the heating circuit is switched off.

The invention is further elucidated below with reference to the drawing, which shows a circuit diagram of an embodiment of a switching circuit according to the invention.

The heating element 1 is through the normally open contact of a relay 2 connected to the supply terminals 3, to which an AC. voltage is applied. By means of a transformer 4, a rectifier 5 and a capacitor 6 a DC. voltage of e.g. 30 v. is derived from the AC. supply voltage. The relay 2, the coil of which is bridged by a capacitor 7, is connected in the anode circuit of a gas discharge valve 8 (e.g. of the PL 21 type), the first control electrode of which is through a grid leak 9 (of e.g. kilohms) connected to the cathode. The heater circuit has not been shown, since this is not essential to the invention.

The first control electrode is also connected through a conventional resistor 10 of e.g. 10 kilohms to the negative terminal of the DC. voltage across the capacitor 6 and is through one or more negative temperature coefi'icient resistors 11 (with an overall resistance at 25 degrees centigrade of e.g. 6 kilohms) connected to the positive terminal of the DC. voltage across the capacitor 6.

The second control electrode is connected to the movable tap of an adjustable potentiometer 12 with a resistance of e.g. 5 kilohms, the ends of which are through conventional resistors 13 and 14 respectively of e.g. 6 kilohms connected to the positive and negative terminals respectively of the DC. voltage across the capacitor 6.

The operation of the circuit has already been described hereinabove.

Instead of a gas discharge valve with heated cathode, as the PL 2.1 type mentioned above, it is also possible to employ a cold cathode gas discharge valve, in which case the heater supply circuit can be dispensed with. The abovementioned resistances and voltage values too have been given by way of example only.

The temperature at which the heating element 1 is switched on and switched off is dependent upon the adjustment of the potentiometer 12. However, it is also possible to dispense with this potentiometer and to make one of the resistors 10, 13 or 14 adjustable.

Whilst a preferred embodiment of the invention has been shown and described above, it will of course, be understood that various other modifications may be made. The appended claims are, therefore, intended to cover any such modification within the true spirit and scope of the invention.

I claim:

1. Switching circuit for an electrically heated blanket, which is controlled by one or more negative temperature coefiicient resistors which are arranged in the blanket, wherein the heating circuit is connected to the supply voltage through the normally open contact of a relay, the coil of which, which is bridged with a capacitor, is connected in the main discharge circuit of a gas discharge valve with two control electrodes, which is supplied with an A.C. voltage, the first control electrode being connected through the negative temperature coeificient resistor(s) to the positive terminal of a D.C. voltage source and being connected through a conventional resistor to the negative terminal thereof, whilst the second control electrode is through conventional resistors connected to the positive and negative terminals respectively of that D.C. voltage source, whilst the first control electrode is also connected to the cathode of the gas discharge valve through a conventional grid leak, the resistance of which is large as compared with the remaining resistors.

2. Switching circuit according to claim 1, wherein one or more of the conventional resistors which are connected to the D.C. voltage source are made adjustable.

References Cited UNITED STATES PATENTS 2,429,453 10/1947 Crowley 219212 2,831,130 4/1958 Obloy 317 132 2,892,917 6/1959 Staats 317-131 X 2,958,008 10/1960 Bray et a1. 2l9501 X LEE T. HIX, Primary Examiner. 

1. SWITCHING CIRCUIT FOR AN ELECTRICALLY HEATED BLANKET, WHICH IS CONTROLLED BY ONE OR MORE NEGATIVE TEMPERATURE COEFFICIENT RESISTORS WHICH ARE ARRANGED IN THE BLANKET, WHEREIN THE HEATING CIRCUIT IS CONNECTED TO THE SUPPLY VOLTAGE THROUGH THE NORMALLY OPEN CONTACT OF A RELAY, THE COIL OF WHICH, WHICH IS BRIDGED WITH A CAPCITOR, IS CONNECTED IN THE MAIN DISCHARGE CIRCUIT OF A GAS DISCHARGE VALVE WITH TWO CONTROL ELECTRODES, WHICH IS SUPPLIED WITH AN A.C. VOLTAGE, THE FIRST CONTROL ELECTRODE BEING CONNECTED THROUGH THE NEGATIVE TEMPERATURE COEFFICIENT RESISTOR(S) TO THE POSITIVE TERMINAL OF A D.C. VOLTAGE SOURCE AND BEING CONNECTED THROUGH A CONVENTIOANL RESISTOR TO THE NEGATIVE TERMINAL THEREOF, WHILST THE SECOND CONTROL ELECTRODE IS THROUGH CONVENTIONAL RESISTORS CONNECTED TO THE POSITIVE AND NEGATIVE TERMINALS RESPECTIVELY OF THAT D.C. VOLTAGE SOURCE, WHILST THE FIRST CONTROL ELECTRODE IS ALSO CONNECTED TO THE CATHODE OF THE GAS DISCHARGE VALVE THROUGH A CONVENTIONAL GRID LEAK, THE RESISTANCE OF WHICHH IS LARGE AS COMPARED WITH THE REMAINING RESISTORS. 